Place a shallow, 9-inch [23-cm.] flan ring on a baking sheet, then line it with the pastry.
Chill.
Place the leeks in a sauce-pan and add the stock.
Simmer the leeks, covered, until they are almost tender.
Drain, reserving the stock.
Saute the sausages until they are browned and almost completely cooked.
Drain them on paper towels.
Melt the butter in a saucepan, then remove from the heat.
Stir in the flour until it is blended into a roux.
Meanwhile, in another saucepan, bring the leeks' reserved cooking stock and the cream to a boil.
Add this stock-and-cream mixture all at once to the roux, stirring with a whisk.
When the sauce thickens, remove it from the heat and beat in the egg yolks.
Return the sauce to the heat and stir it until it is a little thicker, without letting it boil.
Season with the salt and pepper, and the horseradish.
Arrange the cooled leeks in the flan ring.
Pour the sauce over and arrange the sausages evenly on top.
Bake the flan in a preheated 450° F. [230° C] oven for 20 minutes, or until the pastry edges are golden brown.